Some of the MOSFET driving circuits in Flyback converter topology

[Abhilash_P]

BJT Base drive	MOSFET gate drive
Current driven device	Voltage driven device
Current must be applied between the base and emitter terminals to produce a flow of current in the collector	MOSFET produces a flow of current in the drain when a voltage is applied between the gate and source terminals

• Gate Charge:
  • The gate of a MOSFET can be considered to be a capacitance.
  • The gate voltage of a MOSFET does not increase unless its gate input capacitance is charged, and the MOSFET does not turn on until its gate voltage reaches the gate threshold voltage Vth.
  • The gate threshold voltage Vth of a MOSFET is defined as the minimum gate bias required for creating a conduction channel between its source and drain regions.

                                                 

• Input capacitance Ciss = Cgd + Cgs (gate to drain capacitance + gate to source capacitance)
• Output capacitance Coss = Cds + Cgd (drain to source capacitance + gate to drain capacitance)
• Reverse transfer capacitance Crss = Cgd (gate to drain capacitance)

• Calculating Gate drive resistance and Power consumed by the gate drive circuit:
  • Gate drive resistance:

1. Find out the total charge Qtotal (Gate to source and Gate to drain) and Gate driver voltage (Vg) from the datasheet of the MOSFET.
2. Total current through the gate path (coulomb/second) = Qtotal*Switching frequency
3. Gate driver resistance = Vg / (total current).
4. But to reduce gate loss we consider R = T/C, where T = Rise time (from datasheet).

• Power Consumed:

P = Ciss*(Vg)^2*fsw,

Where, Ciss = input capacitance.

            Vg = gate drive supply voltage.

            fsw = switching frequency.

• Different gate driving circuits:

     1. Basic circuit:

• Gate resistor R1: An appropriate resistor value should be selected because it affects the switching speed and therefore the switching loss of a MOSFET.
• PWM: It should be able to meet the following, A gate voltage sufficiently higher than Vth to turn on the MOSFET, voltage sufficiently lower than Vth to turn it off and a drive capability to sufficiently charge the input capacitance.
• Gate resistor R2: This resistor R2 is used to reduce the gate-source voltage to 0V when the input signal is open-circuited.

     2. Push Pull circuit:

• When the parasitic of MOSFET is relatively large and the PWM driving ability is no enough to drive MOSFET normally the below gate driving circuit is used.
• It consists of  Q2, Q3, VCC is used commonly to improve the driving ability.
• This circuit acts as a replacement to the digital logic circuit where the gate voltage is boosted.
• Boosting the logic gate voltage increases the power consumption of the circuit. Hence, we can use this circuit as a replacement.

     3. Accelerating Turn off time (Toff) of the MOSFET:

• During Ton, the gate current flows through R9 which is a high resistance path.
• Resistor R8 and diode D1 offer as low impedance path as possible to discharge the Cgs (gate to source capacitance) quickly when the MOSFET is transition to off state. This can reduce the turn off power loss of the MOSFET.
• Reducing R9 causes voltage spike and ringing effect. Hence a separate discharge path is designed with low impedance.

    4.

• There is one more circuit which is used to accelerate the discharge time of the MOSFET. It is as shown below,
• When the PWM signal has enough driving ability, Q7 is also used to discharge the Cgs (gate to source capacitance) quickly.
• D2 is used to prevent the discharge current flow back to PWM IC and damaging the IC.

     5. Pulse Transformer drive:

• By using a pulse transformer, the need for a separate drive power supply can be eliminated.
• It has a drawback in terms of the power consumption of a drive circuit.
• A pulse transformer is sometimes used to isolate a MOSFET from its driver in order to protect the drive circuit from the MOSFET’s fault.
• A Zener diode is added to quickly reset the transformer.